A receiving device of a communication system (a communication device on the receiving side), may be able to receive signals with an unknown modulation method. Namely, it may be able to determine the modulation method applied to the received signal and carry out demodulation based on the results of the determination. The determination of whether the received signal is multi-carrier modulated or single-carrier modulated (carrier mode determination) is generally performed based on the statistics of the waveform. For example, there are known determination methods that perform the steps of finding the variance for the signal in the frequency domain and determining the carrier mode according to the number of carriers with a variance exceeding a threshold, or determining the carrier mode by detecting the level of similarity to a Gaussian distribution or an arbitrary frequency distribution (e.g., Rayleigh distribution or the like).
In general, in contrast to the time domain waveform of a signal to which a multi-carrier modulation method has been applied (referred to as MC signal hereinafter) that forms a waveform like noise, a signal to which a signal-carrier modulation method has been applied (referred to as SC signal hereinafter) takes on the form modulated by the specific modulation method. Therefore, the carrier mode can be distinguished by investigating the probability distribution of the amplitude. However, if the signal component of the noise increases relative to the desired signal, such as in the case of low SNR (signal to noise power ratio), it is easier to make an erroneous decision in the carrier mode determination using the statistics of the waveform because the waveform approaches a Gaussian distribution to look like an MC signal, even though the waveform is that of an SC signal.